1. Field of the Invention
This invention relates to a copolyester composition and a method for producing a thermo-stable copolyester polymer.
2. Description of the Related Art
Polyesters which have a linear structure, such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), have been widely used for vacuum forming plastic products because of their superior mechanical and optical properties and great chemical resistance. However, the polyesters with linear structure and relatively low molecular weight exhibit deficient melting strength that adversely influences the applicability of the polyesters for manufacture, especially application for a blown film process or a foaming process. To overcome the aforesaid problems, methods of introducing side chains into the linear polyester, i.e., modifying the linear polyester into a branched polyester, with the use of a coupling agent (also known as a chain extender) are conducted such that the molecular weight and the melting strength of the polyester are improved and the resulting branched polyesters are suitable for subsequent processing.
U.S. Pat. No. 6,506,852 discloses a method for producing high molecular weight polyester pellets by heating and pelletizing a polyester mixture. The polyester mixture includes: (a) a linear saturated polyester; (b) a coupling agent composed of a bifunctional epoxide ingredient and a polyfunctional epoxide ingredient; and (c) a coupling reaction catalyst, e.g., a metal salt of a carboxylic acid. The coupling agent is added in the mixture for increasing the swell (degree of swelling) and the amount of long chain branches of the linear saturated polyester so as to produce a polyester product having improved melting strength. The coupling reaction catalyst is added for accelerating the rate of crystallization reaction, thereby leading to a highly polymerized polyester product. In the preferred embodiments of the patent, Al.Na salt of Sasol wax and manganese(II) acetate are respectively used as the catalyst.
In U.S. Pat. No. 6,841,106, the aforesaid polyester mixture is also disclosed to be used for producing a foamed article. The coupling reaction catalysts used in this patent are manganous acetate tetrahydrate, calcium stearate, lithium stearate, or combinations thereof.
However, various problems such as pyrolysis occur during the aforesaid manufacturing process and a resulting polyester product thus made exhibits poor thermostability. The present inventors observed that the species and the amount of the coupling reaction catalyst may influence the polymerization reaction and the physical property of the resulting polyester product. When a carboxylate of sodium or a carboxylate of manganese is used as the catalyst, the catalyst efficiency is undesirable such that the polymerization reaction is likely to be incomplete, thereby resulting in a poor melting strength of the resulting polyester product.
It is desired in the art to provide a catalyst suitable for polymerization reaction of a polyester or a copolyester composition, which may allow the polymerization reaction to proceed efficiently, thereby obtaining a resulting polyester product with superior physical properties.